The amino acid transport system b[sup o,+] and cystinuria.

Amino acid transport in mammalian plasma membranes is mediated by a multiplicity of amino acid transport systems. Some of them (systems L, y[sup +]L, x[sub c][sup -] and b[sup o,+]) are the result of the activity of heteromeric amino acid transporters (HAT) (i.e. transport activity is elicited by the coexpression of a heavy and a light subunit). The two heavy subunits known today (HSHAT: rBAT and 4F2hc) were identified in 1992, and light subunits (LSHAT: LAT-1, LAT-2, asc-1, y[sup +]LAT-1, y[sup +]LAT-2, xCT and b[sup o,+]AT) have been cloned in the last 2 years. Defects in two genes of this family (SLC3A1, encoding rBAT and SLC7A9, encoding b[sup o,+]AT) are responsible for cystinuria, an inherited aminoaciduria of cystine and dibasic amino acids. This finding and functional studies of rBAT and b[sup o,+]AT suggested that these two proteins encompass ed the high-affinity renal reabsorption system of cystine. In contrast to this view, immunofluorescence studies showed that rBAT is most abundant in the proximal straight tubule, and b[sup o,+]AT is most abundant in the proximal convoluted tubule of the nephron. The need for a newlight subunit for rBAT and a heavy subunit for b[sup o,+]AT is discussed. [ABSTRACT FROM AUTHOR]

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